1. Field of the Invention
The present invention relates to a zoom lens system having at least three lens groups, and also relates to a focus adjustment method for such a zoom lens system.
2. Description of the Prior Art
In a zoom lens system, a zoom adjustment by which (i) a position of an image plane does not move upon zooming, and (ii) the image plane coincides (within an allowable tolerance) with the film surface (the image-forming plane) is essential.
More specifically, the above (i) is referred to as a zoom adjustment in the narrower sense (hereinafter, an narrower zoom adjustment); and the above (ii) is referred to as a back-focus adjustment.
The term “focus adjustment” (a part of the title of the present invention) is generally understood as a zoom adjustment in the broader sense (hereinafter, a broader zoom adjustment) which includes the concepts of the narrower zoom adjustment and the back-focus adjustment.
In the prior art, the broader zoom adjustment (i.e., the focus adjustment) has been performed by mechanically adjusting the positions of the lens groups (focus-adjusting lens groups), when the zoom lens system is being assembled.
Furthermore, in the focus adjustment (the broader zoom adjustment) of the prior art, the movement of the focal point at both the short and long focal length extremities is mechanically corrected by using two focus-adjusting lens groups at two respective positions. However, the movement of the focal point at an intermediate focal length cannot be adequately corrected. Particularly, in a zoom lens system which requires a high resolution, the movement of the focal point at an intermediate focal length has become a problem to be solved.
On the other hand, in recent years, in optical systems which are used in digital cameras, etc., a rear-focusing type lens system in which the focusing lens group is light in weight, or an internal-focusing type lens system, has often been employed. In these types of focusing, the position of the focusing lens group is pulse-controlled. According to a pulse-control process, even a variable-focus lens system, in which the position of the focal point changes with a change in focal length, can be utilized in the same way as a zoom lens system is used, by memorizing an in-focus object distance per focal length.
However, if a focusing function and a correcting-function for the focal-point movement are provided in the focusing lens group of a variable-focus lens system, the traveling distance of the focusing lens group becomes longer, so that the overall length of the lens system becomes longer. Furthermore, if the broader zoom adjustment (the focus adjustment) is carried out by one focus-adjusting lens group, the distance between the focus-adjusting lens group and a lens group immediately in front of the focus-adjusting lens group needs to be made longer; likewise, the distance between the focus-adjusting lens group and a lens group immediately behind the focus-adjusting lens group needs to be made longer. These distances are necessary for securing a space for the broader zoom adjustment (the focus adjustment). Consequently, it becomes difficult to attain further miniaturization of the lens system. In particular, if the distance between the first lens group and the second lens group becomes longer, the diameter of the first lens group becomes undesirably larger.
The applicant of the present invention has proposed a focus-adjusting method, in Japanese Unexamined Patent Publication No. 2000-352649, which reduces a focus shift at an intermediate focal length. According to the focus-adjusting method of the above-mentioned publication, it becomes possible to mechanically adjust the positions of at least three lens groups, and to calculate the amount of correction based on the amount of movement of the focal point at three focal lengths at least.
The focus-adjusting method mentioned above can make a focus shift after the broader zoom adjustment (the focus adjustment) smaller; however, no consideration has been taken to the traveling distance and direction of the focus-adjusting lens groups. Therefore the distance between the focus-adjusting lens groups has to be made longer in order to prevent the focus-adjusting lens groups from interfering with each other. As a result, the overall length of the lens system tends to be longer.